Whitaker’s commentary on the Mueller probe is clearly sympathetic to Trump — to the extent of musing about defunding Mueller and even suggesting the Trump Tower meeting was business-as-usual. If he continues as acting attorney general, he could rule on consequential issues including potential indictments of Trump allies, up to and including the president’s son. He could reject an attempt to subpoena the president.

His fate now appears to rest, in part, on an obscure century-old Supreme Court case that isn’t exactly analogous to Whitaker’s situation. But it may not need to be, and the ultimate arbiter of this could be the Supreme Court.

The basic issue at hand is whether Whitaker, who did not need to be confirmed by the Senate for his previous job as then-Attorney General Jeff Sessions’s chief of staff, can replace Sessions in an acting capacity. The Constitution’s “Advice and Consent” clause holds that principal officers — that is, those who have no superior but the president, of which the attorney general is one — must have Senate approval. Since Whitaker would be serving in place of a principal officer, George Conway and former solicitor general Neal Katyal wrote in the New York Times, his appointment is “illegal.”

Unless, that is, he wouldn’t actually be considered a principal officer. Senate Judiciary Committee Chairman Charles E. Grassley (R-Iowa) has said in recent days that Whitaker has defended himself by citing a 2003 opinion from the Justice Department’s Office of Legal Counsel. That 2003 opinion (which was incidentally written by Ed Whelan, most recently of Brett M. Kavanaugh-related infamy) in turn relies upon an 1898 Supreme Court case called United States v. Eaton.

Eaton seemed to lower the bar for acting officials serving in place of principals. It addressed the case of an unconfirmed “vice consul” serving in Siam — what is now Thailand. That vice consul was appointed to serve as a temporary replacement for the Senate-confirmed consul when the consul fell ill, but then a compensation dispute arose, with the ill consul and the vice consul both claiming the consul’s salary.

The Supreme Court ruled that the vice consul could act for the consul, even though he hadn’t been confirmed by the Senate, because he wasn’t technically becoming the consul:

Because the subordinate officer is charged with the performance of the duty of the superior for a limited time, and under special and temporary conditions, he is not thereby transformed into the superior and permanent official. To so hold would render void any and every delegation of power to an inferior to perform under any circumstances or exigency the duties of a superior officer, and the discharge of administrative duties would be seriously hindered.

The parallel to Whitaker’s case is apparent. Just as the vice consul didn’t need to be confirmed because he was only serving on an acting basis, Whitaker seems likely to argue that he, too, can serve without confirmation on a temporary basis. He’ll essentially argue that the acting job isn’t a principal job, because it’s temporary. And he’ll have some backup, given that acting officials don’t assume the salary of the principals they are filling in for.

But the parallels only go so for. In Eaton, the vice consul was the highest-ranking official available in Siam; the next person in line had not yet been confirmed by the Senate and hadn’t made his way overseas. In the case of Whitaker, there are Senate-confirmed officials in place, including most notably Rosenstein but also Solicitor General Noel Francisco. There is even a law on the books that determines the line of succession.

The language above from Eaton also cites “special and temporary conditions.” Trump effectively forced Sessions out, and even though Sessions technically resigned, he took care to note it was at Trump’s urging. “As you requested,” Sessions wrote, “I am submitting my resignation.” So is it an emergency if it’s a situation of Trump’s own making? If he can do this, couldn’t he just force out people he doesn’t like in the name of creating supposed emergencies and appointing unconfirmed acting replacements?

Anne Joseph O’Connell, an expert on presidential appointments at Stanford University, said she believes Whitaker’s appointment is constitutional — even if it’s “terrible as a matter of policy.”

“The Whitaker situation is hard as it doesn’t appear to be an emergency. But such facts can make bad law,” she said. “What if Sessions were pushed out for true wrongdoing? Would that have counted? I don’t think courts want to get in the business of figuring out reasons for departures.”

The situation might not even need to be an emergency, though. At the time of Eaton in 1898, there was no such thing as the Federal Vacancies Reform Act. That act, which came exactly 100 years later, in 1998, created three means by which a Senate-confirmed vacancy could be filled on a temporary basis. The third method is by someone with at least 90 days' service in the last year at the agency at issue, provided they have served above a certain pay grade, known as “GS-15″ — qualifications that Whitaker meets.

The FVRA makes no mention of an “emergency” being required, though. Instead, it says one of the three options can be triggered as long as the principal “dies, resigns, or is otherwise unable to perform the functions and duties of the office.” Sessions technically resigned, so as long as the FVRA is considered constitutional, Whitaker would seem to be in the clear.

“It’s true that the Whitaker situation is different from that of our 1890s man in Siam,” said Andrew Rudalevige of Bowdoin College, “but so is the law.”

The counterargument to all of that comes from John Yoo, a Bush administration aide who served in the DOJ’s Office of Legal Counsel around the same time as the 2003 opinion and previously served as general counsel for the Senate Judiciary Committee. He writes in the Atlantic that Whitaker satisfies the requirements of the FVRA, but that the appointment might still be deemed unconstitutional:

Defenders might rely on an 1898 Supreme Court decision, United States v. Eaton, which allowed for the temporary appointment of a vice-consul in Thailand “for a limited time, and under special and temporary conditions,” namely, the illness of the consul and the vast distance between the U.S. and Thailand. This is basically the approach of a 2003 Justice Department opinion approving the elevation of an assistant Office of Management and Budget director to acting director, and the likely reasoning of the White House in appointing Whitaker.

...

Elevating practical needs over constitutional meaning, however, is not an attitude usually adopted by conservatives. ... It is difficult to see [Chief Justice] John Roberts’s Court finding that the acting attorney general, responsible for all federal law enforcement in the nation, amounts to the same sort of officer as a vice-consul struggling in the hinterlands of Thailand before the days of air flight and instantaneous communications.

But David Lewis of Vanderbilt University said the court was unlikely to make such an exception.

“In cases where the Constitution is silent like this, Congress and the president enact laws to regularize practice,” Lewis said. “The 1998 Vacancies Reform Act attempted to regularize the presidential practice of filling Senate confirmed positions on a temporary basis and did so with an eye toward curbing the president’s ability to use acting officials as replacements for actual nominees.”

The arguments against Whitaker’s appointment seem more aspirational and idealistic -- as if that’s what the proponents think the courts should decide and not necessarily what they will. That’s not necessarily from a partisan perspective (Conway and Yoo are both Republicans) but from a Constitutional one. But on the other side, experts on appointments seem to see Whitaker as likely to prevail. We shall see. The implications are massive.